dead bees around the box and inside the water
got the nucleus three days ago
Have you observed the bees flying? It may be that the nuc is being robbed by a stronger colony nearby, and triggered as the spring flow fades. Robbing = fighting.
If robbing is underway
do not feed the nuc, but close and move it in the evening to a site three miles away you can borrow for a couple of weeks - a friend, a farmer or old Frank's field. When the blackberry opens robbing will stop and you can bring your nuc home.
The box & three days is not a factor: we've parked nucs in the green Correx transit boxes for weeks without trouble, provided all vent holes were punched out and the boxes had shade for part of the day. Some beekeepers keep bees in them all summer.
any specific thing I should do today?
Watch the behaviour of the bees. If you transfer the nuc to a hive reduce the entrance to one bee space until you know whether robbing is the cause of mortality, and for the same reason
do not feed.
Check the stores frames, take a photo of the sealed stores and post it here. Bees will have foraged from the moment you opened the box entrance three days ago, so check to see fresh nectar in cells.
Bees will find their own source of water and there is no need to provide it (you're in Basingstoke, not Southern Portugal).
There is an outside chance that the colony is diseased or has foraged on local plants sprayed with insecticide, so take photos of the brood combs and the dead bees and post here.
